The methanogenic degradation of subsurface carbonaceous material is of significant commercial interest for a variety of reasons including production of natural gas (including methane). Methane is a predominant end-product of anaerobic microbially-mediated organic-matter decomposition following a variety of carbon-pathways and intermediate steps.
Recent technological advances have enabled characterization of microbial communities and the biogeochemical processes that take place in the subsurface. These processes generally occur under non-ideal conditions due to limiting nutrients and sub-optimal microbial community structure. Under normal sub-surface conditions, microbial gas formed in these natural “bioreactors” is generated at very slow rates due to limited nutrients and/or other environmental conditions, e.g., suboptimal water chemistry, pH, salinity and the like.
Organisms that utilize oxygen as a final electron acceptor during metabolism recover a significant amount of energy. These aerobic organisms often possess all of the required enzyme-encoding genes to perform complete organic transformations singlehandedly. In the absence of oxygen, however, organisms that respire requiring alternative electron acceptors such as nitrate, sulfate, Fe (III), and the like, or that ferment substrates recover significantly less energy than their aerobic counterparts. As such microbes that inhabit anaerobic environments tend to specialize in specific biochemical reactions and require other specialized organisms to complete an organic transformation (Schink 1997), these organisms often have syntrophic relationships with other organisms where each member is dependent on the other for the exchange of intermediates and their contributions are tightly linked (McInerney, Sieber et al. 2009).
The most commonly described subsurface methanogenic pathways of microbially-generated methane (biogas) formation are CO2-reduction and acetate fermentation (Faiz and Hendry, 2009; Dolfing et al., 2008, Strapoć et al., 2008, Brooks Avery et al., 2003; Budwill, 2003; Whiticar et al., 1986). Different optimal pH ranges have been identified for coal conversion to methane via lab bench methanogenic enrichments (Green, et al., 2008; Srivastava and Walia, 1998). Gas production data from treated CBM wells Powder River Basin shows likely increase of gas production after huff and puff treatments of phosphorus and acetate. Presence of methyl/methanol utilizing methanogens, i.e. Methanolobus, has been recently reported in coal-bearing subsurface environments (Mochimaru et al., 2007, Doerfert et al., 2009).
Volkwein, U.S. Pat. No. 5,424,195, uses a consortium of microorganisms for in situ biological conversion of higher rank coals to methane. Srivastava and Walia, U.S. Pat. No. 5,854,032, describe coal treatment with a culture of microorganisms which act upon the coal to produce humic acids, methane, volatile fatty acids, and lower alcohols. Menger et al., U.S. Pat. No. 6,143,534, uses ligninase to assist in the biochemical reaction of lignin substrates such as coal. Converse et al., U.S. Pat. No. 6,543,535, stimulate the activity of microbial consortia in a hydrocarbon-bearing, subterranean formation to convert hydrocarbons to methane and other hydrocarbon gases, which can be produced. Scott and Guyer, US2004033557, modify and adjust the bacterial consortia and/or nutrients to maximize bacterial degradation of the organic matter and subsequent generation of methane, hydrogen, carbon dioxide, and other gases. Larter and associates, US20070251146, provide a process for stimulating microbial methane production in a petroleum-bearing subterranean formation. Pfeiffer et al., U.S. Pat. No. 7,426,960, stimulate biogenic production of a metabolite with enhanced hydrogen content including the steps of forming an opening in a geologic formation to provide access to a consortium of microorganisms, and injecting water into the opening to disperse at least a portion of the consortium over a larger region of a hydrocarbon deposit. Toledo et al., US20100047793, use nucleic acid information obtained from a variety of microorganisms within the hydrocarbon-bearing formation to identify enzymes present in the microorganisms that function in a variety of pathways converting a portion of the hydrocarbon source to methane. Pfeiffer et al., U.S. Pat. No. 7,696,132, also use a combination of hydrogen and phosphorous compounds to stimulate a consortium of microorganisms to metabolize carbonaceous material into a metabolic product with enhanced hydrogen content. Jin et al., US2011027849 provide microbial population stimulation amendments, indiscriminate microbial population stimulation amendments, additional microbial population stimulation amendments, sulfate reduction competition shield amendments, predetermined microbial population stimulation amendments, and the like which can be introduced into various hydrocarbon-bearing formations to enhance the production of biogenic methane. Gates et al., WO2010012093, feature a method for producing biogenerated gas from a zone in a reservoir.
Unfortunately, dominant methanogenic pathways involved in conversion of carbonaceous material to methane are required to improve methanogenic activity in situ. Therefore, chemical and microbial amendments if used were not optimally targeted for the most favorable methanogenic pathway(s) in the microbial communities with the substrate(s) present, so methanogenesis was not maximized. Furthermore, biogas production customized for a given carbonaceous substrate and aqueous environment must also be combined with further technical evaluation minimizing the risks and potential adverse effects of the targeted process. What is needed is a method of identifying the optimum microbial consortium for methanogenesis under formation conditions, understanding the risks associated with each method of microbially enhanced methane production, and optimization of those methods for each reservoir, substrate, and/or consortium of microbes.